Hydraulic fracturing is the primary technique for accessing shale-bound hydrocarbons. Commonly, water with small concentrations of friction reducer—“slickwater”—is used to fracture tight gas formations with very low permeability, such as shale. Typically, after wellbore formation, slickwater is injected at high pressure and flow rate into the wellbore to form fractures in hydrocarbon-bearing shale formations through which the hydrocarbons are extracted. Once the fluid flows back from the subsurface, gas is free to flow out of the formation rock to surface production infrastructure.
Aqueous fracturing fluids generally contain proppant and chemical additives. Most commonly, polyacrylamides are incorporated as friction reducers to permit sustained injection pressure of the fracturing fluid at depth. The industry standard for reducing pipe friction pressure in hydraulic fracturing is to use either a polyacrylamide friction reducer emulsified in oil, or a variation of this, including acrylamide copolymers or granulated polyacrylamides. Generally, polyacrylamide is added “on the fly” and in a concentration on the order of one gallon or part per thousand to create the ubiquitous “slickwater” in fracturing operations.
Prior to hydrocarbon production, efforts are made to back-produce water used for hydraulic fracturing. Typically, however, only a fraction of the original fracturing water is recovered in back-production. The remaining fluid interferes with hydrocarbon production due to the presence of water within the fractures acting as a hydrocarbon flow barrier, as well as shale clay hydration and concomitant swelling, which results in fracture closure and proppant embedment.
Furthermore, the continued expansion of hydraulic fracturing activity—especially in slickwater operations—has given rise to concerns regarding water quantity and quality. One of the most pressing issues facing the fracturing industry is the dilemma of sourcing water for operations and treating or disposing of the water returned after injection. On one hand, operators in arid areas must compete with other users to obtain enough fresh water to perform fracturing operations, while in other areas the flowback water after a treatment must be either treated or disposed of in injection wells, often at great cost.
Reuse of saline formation waters from flowback can help to alleviate both of these problems. However, the formation waters that contain concentrated and difficult-to-remove salt ions—especially divalent cations—cannot be used with typical polyacrylamide friction reducers, due to these additives' dramatically decreased effectiveness in such fluids. Onsite treatment of flowback water is often expensive, and blending of back-produced fracking fluids with fresh water is also an undesirable option, since the contaminants returned with flowback water can detrimentally affect conventional friction reducers that are affected by salt ions. Without friction reduction, the enormous flow rates needed to suspend proppant and fracture a shale formation cannot be achieved safely nor economically.